ssh_config • man page

ssh_config - OpenSSH SSH client configuration files

ssh_config • man page

ssh_config - OpenSSH SSH client configuration files

ssh_config (5)

Leading comments

Author: Tatu Ylonen <ylo@cs.hut.fi>
Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
All rights reserved
As far as I am concerned, the code I have written for this software
can be used freely for any purpose. Any derived versions of this
software must be clearly marked as such, and if the derived work is
incompatible with the protocol description in the RFC file, it must be
called by a name other than "ssh" or "Secure Shell".
Copyright (c) 1999,2000 Markus ...

(The comments found at the beginning of the groff file "man5/ssh_config.5".)

NAME

ssh_config
- OpenSSH SSH client configuration files

SYNOPSIS

~/.ssh/config/etc/ssh/ssh_config

DESCRIPTION

ssh(1)
obtains configuration data from the following sources in
the following order:

command-line options

user's configuration file
(~/.ssh/config
)

system-wide configuration file
(/etc/ssh/ssh_config
)

For each parameter, the first obtained value
will be used.
The configuration files contain sections separated by
``Host''
specifications, and that section is only applied for hosts that
match one of the patterns given in the specification.
The matched host name is usually the one given on the command line
(see the
CanonicalizeHostname
option for exceptions.)

Since the first obtained value for each parameter is used, more
host-specific declarations should be given near the beginning of the
file, and general defaults at the end.

Note that the Debian
openssh-client
package sets several options as standard in
/etc/ssh/ssh_config
which are not the default in
ssh(1):

SendEnv LANG LC_*

HashKnownHosts yes

GSSAPIAuthentication yes

The configuration file has the following format:

Empty lines and lines starting with
`#'
are comments.
Otherwise a line is of the format
``keyword arguments''
Configuration options may be separated by whitespace or
optional whitespace and exactly one
`='
;
the latter format is useful to avoid the need to quote whitespace
when specifying configuration options using the
sshscp
and
sftp
-o
option.
Arguments may optionally be enclosed in double quotes
()
in order to represent arguments containing spaces.

The possible
keywords and their meanings are as follows (note that
keywords are case-insensitive and arguments are case-sensitive):

Host

Restricts the following declarations (up to the next
Host
or
Match
keyword) to be only for those hosts that match one of the patterns
given after the keyword.
If more than one pattern is provided, they should be separated by whitespace.
A single
`*'
as a pattern can be used to provide global
defaults for all hosts.
The host is usually the
hostname
argument given on the command line
(see the
CanonicalizeHostname
option for exceptions.)

A pattern entry may be negated by prefixing it with an exclamation mark
(`!'
)
If a negated entry is matched, then the
Host
entry is ignored, regardless of whether any other patterns on the line
match.
Negated matches are therefore useful to provide exceptions for wildcard
matches.

See
Sx PATTERNS
for more information on patterns.

Match

Restricts the following declarations (up to the next
Host
or
Match
keyword) to be used only when the conditions following the
Match
keyword are satisfied.
Match conditions are specified using one or more criteria
or the single token
all
which always matches.
The available criteria keywords are:
canonicalexechostoriginalhostuser
and
localuser
The
all
criteria must appear alone or immediately after
canonical
Other criteria may be combined arbitrarily.
All criteria but
all
and
canonical
require an argument.
Criteria may be negated by prepending an exclamation mark
(`!'
)

The
canonical
keyword matches only when the configuration file is being re-parsed
after hostname canonicalization (see the
CanonicalizeHostname
option.)
This may be useful to specify conditions that work with canonical host
names only.
The
exec
keyword executes the specified command under the user's shell.
If the command returns a zero exit status then the condition is considered true.
Commands containing whitespace characters must be quoted.
The following character sequences in the command will be expanded prior to
execution:
`%L'
will be substituted by the first component of the local host name,
`%l'
will be substituted by the local host name (including any domain name),
`%h'
will be substituted by the target host name,
`%n'
will be substituted by the original target host name
specified on the command-line,
`%p'
the destination port,
`%r'
by the remote login username, and
`%u'
by the username of the user running
ssh(1).

The other keywords' criteria must be single entries or comma-separated
lists and may use the wildcard and negation operators described in the
Sx PATTERNS
section.
The criteria for the
host
keyword are matched against the target hostname, after any substitution
by the
Hostname
or
CanonicalizeHostname
options.
The
originalhost
keyword matches against the hostname as it was specified on the command-line.
The
user
keyword matches against the target username on the remote host.
The
localuser
keyword matches against the name of the local user running
ssh(1)
(this keyword may be useful in system-wide
~/.ssh/config
files).

AddKeysToAgent

Specifies whether keys should be automatically added to a running
ssh-agent1.
If this option is set to
``yes''
and a key is loaded from a file, the key and its passphrase are added to
the agent with the default lifetime, as if by
ssh-add1.
If this option is set to
``ask''
ssh
will require confirmation using the
SSH_ASKPASS
program before adding a key (see
ssh-add1
for details).
If this option is set to
``confirm''
each use of the key must be confirmed, as if the
-c
option was specified to
ssh-add1.
If this option is set to
``no''
no keys are added to the agent.
The argument must be
``yes''
``confirm''
``ask''
or
``no''
The default is
``no''

AddressFamily

Specifies which address family to use when connecting.
Valid arguments are
``any''
``inet''
(use IPv4 only), or
``inet6''
(use IPv6 only).
The default is
``any''

BatchMode

If set to
``yes''
passphrase/password querying will be disabled.
In addition, the
ServerAliveInterval
option will be set to 300 seconds by default.
This option is useful in scripts and other batch jobs where no user
is present to supply the password,
and where it is desirable to detect a broken network swiftly.
The argument must be
``yes''
or
``no''
The default is
``no''

BindAddress

Use the specified address on the local machine as the source address of
the connection.
Only useful on systems with more than one address.
Note that this option does not work if
UsePrivilegedPort
is set to
``yes''

CanonicalDomains

When
CanonicalizeHostname
is enabled, this option specifies the list of domain suffixes in which to
search for the specified destination host.

CanonicalizeFallbackLocal

Specifies whether to fail with an error when hostname canonicalization fails.
The default,
``yes''
will attempt to look up the unqualified hostname using the system resolver's
search rules.
A value of
``no''
will cause
ssh(1)
to fail instantly if
CanonicalizeHostname
is enabled and the target hostname cannot be found in any of the domains
specified by
CanonicalDomains

CanonicalizeHostname

Controls whether explicit hostname canonicalization is performed.
The default,
``no''
is not to perform any name rewriting and let the system resolver handle all
hostname lookups.
If set to
``yes''
then, for connections that do not use a
ProxyCommandssh(1)
will attempt to canonicalize the hostname specified on the command line
using the
CanonicalDomains
suffixes and
CanonicalizePermittedCNAMEs rules.
If
CanonicalizeHostname
is set to
``always''
then canonicalization is applied to proxied connections too.

If this option is enabled, then the configuration files are processed
again using the new target name to pick up any new configuration in matching
Host
and
Match
stanzas.

CanonicalizeMaxDots

Specifies the maximum number of dot characters in a hostname before
canonicalization is disabled.
The default,
``1''
allows a single dot (i.e. hostname.subdomain).

CanonicalizePermittedCNAMEs Specifies rules to determine whether CNAMEs should be followed when

canonicalizing hostnames.
The rules consist of one or more arguments of
source_domain_list : target_domain_list
where
source_domain_list
is a pattern-list of domains that may follow CNAMEs in canonicalization,
and
target_domain_list
is a pattern-list of domains that they may resolve to.

For example,
``*.a.example.com:*.b.example.com,*.c.example.com''
will allow hostnames matching
``*.a.example.com''
to be canonicalized to names in the
``*.b.example.com''
or
``*.c.example.com''
domains.

CertificateFile

Specifies a file from which the user's certificate is read.
A corresponding private key must be provided separately in order
to use this certificate either
from an
IdentityFile
directive or
-i
flag to
ssh(1),
via
ssh-agent1,
or via a
PKCS11Provider

The file name may use the tilde
syntax to refer to a user's home directory or one of the following
escape characters:
`%d'
(local user's home directory),
`%u'
(local user name),
`%l'
(local host name),
`%h'
(remote host name) or
`%r'
(remote user name).

It is possible to have multiple certificate files specified in
configuration files; these certificates will be tried in sequence.
Multiple
CertificateFile
directives will add to the list of certificates used for
authentication.

ChallengeResponseAuthentication

Specifies whether to use challenge-response authentication.
The argument to this keyword must be
``yes''
or
``no''
The default is
``yes''

CheckHostIP

If this flag is set to
``yes''
ssh(1)
will additionally check the host IP address in the
known_hosts
file.
This allows ssh to detect if a host key changed due to DNS spoofing
and will add addresses of destination hosts to
~/.ssh/known_hosts
in the process, regardless of the setting of
StrictHostKeyChecking
If the option is set to
``no''
the check will not be executed.
The default is
``yes''

Cipher

Specifies the cipher to use for encrypting the session
in protocol version 1.
Currently,
``blowfish''
``3des''
and
``des''
are supported.
des
is only supported in the
ssh(1)
client for interoperability with legacy protocol 1 implementations
that do not support the
3des
cipher.
Its use is strongly discouraged due to cryptographic weaknesses.
The default is
``3des''

Ciphers

Specifies the ciphers allowed for protocol version 2
in order of preference.
Multiple ciphers must be comma-separated.
If the specified value begins with a
`+'
character, then the specified ciphers will be appended to the default set
instead of replacing them.

The list of available ciphers may also be obtained using the
-Q
option of
ssh(1)
with an argument of
``cipher''

ClearAllForwardings

Specifies that all local, remote, and dynamic port forwardings
specified in the configuration files or on the command line be
cleared.
This option is primarily useful when used from the
ssh(1)
command line to clear port forwardings set in
configuration files, and is automatically set by
scp(1)
and
sftp(1).
The argument must be
``yes''
or
``no''
The default is
``no''

Compression

Specifies whether to use compression.
The argument must be
``yes''
or
``no''
The default is
``no''

CompressionLevel

Specifies the compression level to use if compression is enabled.
The argument must be an integer from 1 (fast) to 9 (slow, best).
The default level is 6, which is good for most applications.
The meaning of the values is the same as in
gzip(1).
Note that this option applies to protocol version 1 only.

ConnectionAttempts

Specifies the number of tries (one per second) to make before exiting.
The argument must be an integer.
This may be useful in scripts if the connection sometimes fails.
The default is 1.

ConnectTimeout

Specifies the timeout (in seconds) used when connecting to the
SSH server, instead of using the default system TCP timeout.
This value is used only when the target is down or really unreachable,
not when it refuses the connection.

ControlMaster

Enables the sharing of multiple sessions over a single network connection.
When set to
``yes''
ssh(1)
will listen for connections on a control socket specified using the
ControlPath
argument.
Additional sessions can connect to this socket using the same
ControlPath
with
ControlMaster
set to
``no''
(the default).
These sessions will try to reuse the master instance's network connection
rather than initiating new ones, but will fall back to connecting normally
if the control socket does not exist, or is not listening.

Setting this to
``ask''
will cause ssh
to listen for control connections, but require confirmation using
ssh-askpass1.
If the
ControlPath
cannot be opened,
ssh will continue without connecting to a master instance.

X11 and
ssh-agent1
forwarding is supported over these multiplexed connections, however the
display and agent forwarded will be the one belonging to the master
connection i.e. it is not possible to forward multiple displays or agents.

Two additional options allow for opportunistic multiplexing: try to use a
master connection but fall back to creating a new one if one does not already
exist.
These options are:
``auto''
and
``autoask''
The latter requires confirmation like the
``ask''
option.

ControlPath

Specify the path to the control socket used for connection sharing as described
in the
ControlMaster
section above or the string
``none''
to disable connection sharing.
In the path,
`%L'
will be substituted by the first component of the local host name,
`%l'
will be substituted by the local host name (including any domain name),
`%h'
will be substituted by the target host name,
`%n'
will be substituted by the original target host name
specified on the command line,
`%p'
the destination port,
`%r'
by the remote login username,
`%u'
by the username and
`%i'
by the numeric user ID (uid) of the user running
ssh(1),
and
`%C'
by a hash of the concatenation: %l%h%p%r.
It is recommended that any
ControlPath
used for opportunistic connection sharing include
at least %h, %p, and %r (or alternatively %C) and be placed in a directory
that is not writable by other users.
This ensures that shared connections are uniquely identified.

ControlPersist

When used in conjunction with
ControlMaster
specifies that the master connection should remain open
in the background (waiting for future client connections)
after the initial client connection has been closed.
If set to
``no''
then the master connection will not be placed into the background,
and will close as soon as the initial client connection is closed.
If set to
``yes''
or
``0''
then the master connection will remain in the background indefinitely
(until killed or closed via a mechanism such as the
ssh(1)
``-O exit
''
option).
If set to a time in seconds, or a time in any of the formats documented in
sshd_config5,
then the backgrounded master connection will automatically terminate
after it has remained idle (with no client connections) for the
specified time.

DynamicForward

Specifies that a TCP port on the local machine be forwarded
over the secure channel, and the application
protocol is then used to determine where to connect to from the
remote machine.

The argument must be
[bind_address : port
]
IPv6 addresses can be specified by enclosing addresses in square brackets.
By default, the local port is bound in accordance with the
GatewayPorts
setting.
However, an explicit
bind_address
may be used to bind the connection to a specific address.
The
bind_address
of
``localhost''
indicates that the listening port be bound for local use only, while an
empty address or
`*'
indicates that the port should be available from all interfaces.

Currently the SOCKS4 and SOCKS5 protocols are supported, and
ssh(1)
will act as a SOCKS server.
Multiple forwardings may be specified, and
additional forwardings can be given on the command line.
Only the superuser can forward privileged ports.

EnableSSHKeysign

Setting this option to
``yes''
in the global client configuration file
/etc/ssh/ssh_config
enables the use of the helper program
ssh-keysign8
during
HostbasedAuthentication
The argument must be
``yes''
or
``no''
The default is
``no''
This option should be placed in the non-hostspecific section.
See
ssh-keysign8
for more information.

EscapeChar

Sets the escape character (default:
`~'
) .
The escape character can also
be set on the command line.
The argument should be a single character,
`^'
followed by a letter, or
``none''
to disable the escape
character entirely (making the connection transparent for binary
data).

ExitOnForwardFailure

Specifies whether
ssh(1)
should terminate the connection if it cannot set up all requested
dynamic, tunnel, local, and remote port forwardings, (e.g.
if either end is unable to bind and listen on a specified port).
Note that
ExitOnForwardFailure
does not apply to connections made over port forwardings and will not,
for example, cause
ssh(1)
to exit if TCP connections to the ultimate forwarding destination fail.
The argument must be
``yes''
or
``no''
The default is
``no''

FingerprintHash

Specifies the hash algorithm used when displaying key fingerprints.
Valid options are:
``md5''
and
``sha256''
The default is
``sha256''

ForwardAgent

Specifies whether the connection to the authentication agent (if any)
will be forwarded to the remote machine.
The argument must be
``yes''
or
``no''
The default is
``no''

Agent forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(for the agent's Unix-domain socket)
can access the local agent through the forwarded connection.
An attacker cannot obtain key material from the agent,
however they can perform operations on the keys that enable them to
authenticate using the identities loaded into the agent.

ForwardX11

Specifies whether X11 connections will be automatically redirected
over the secure channel and
DISPLAY
set.
The argument must be
``yes''
or
``no''
The default is
``no''

X11 forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(for the user's X11 authorization database)
can access the local X11 display through the forwarded connection.
An attacker may then be able to perform activities such as keystroke monitoring
if the
ForwardX11Trusted
option is also enabled.

ForwardX11Timeout

Specify a timeout for untrusted X11 forwarding
using the format described in the
TIME FORMATS section of
sshd_config5.
X11 connections received by
ssh(1)
after this time will be refused.
The default is to disable untrusted X11 forwarding after twenty minutes has
elapsed.

ForwardX11Trusted

If this option is set to
``yes''
remote X11 clients will have full access to the original X11 display.

If this option is set to
``no''
remote X11 clients will be considered untrusted and prevented
from stealing or tampering with data belonging to trusted X11
clients.
Furthermore, the
xauth(1)
token used for the session will be set to expire after 20 minutes.
Remote clients will be refused access after this time.

The default is
``yes''
(Debian-specific).

See the X11 SECURITY extension specification for full details on
the restrictions imposed on untrusted clients.

GatewayPorts

Specifies whether remote hosts are allowed to connect to local
forwarded ports.
By default,
ssh(1)
binds local port forwardings to the loopback address.
This prevents other remote hosts from connecting to forwarded ports.
GatewayPorts
can be used to specify that ssh
should bind local port forwardings to the wildcard address,
thus allowing remote hosts to connect to forwarded ports.
The argument must be
``yes''
or
``no''
The default is
``no''

GlobalKnownHostsFile

Specifies one or more files to use for the global
host key database, separated by whitespace.
The default is
/etc/ssh/ssh_known_hosts
/etc/ssh/ssh_known_hosts2

GSSAPIAuthentication

Specifies whether user authentication based on GSSAPI is allowed.
The default is
``no''

GSSAPIKeyExchange

Specifies whether key exchange based on GSSAPI may be used. When using
GSSAPI key exchange the server need not have a host key.
The default is
``no''

GSSAPIClientIdentity

If set, specifies the GSSAPI client identity that ssh should use when
connecting to the server. The default is unset, which means that the default
identity will be used.

GSSAPIServerIdentity

If set, specifies the GSSAPI server identity that ssh should expect when
connecting to the server. The default is unset, which means that the
expected GSSAPI server identity will be determined from the target
hostname.

GSSAPIDelegateCredentials

Forward (delegate) credentials to the server.
The default is
``no''

GSSAPIRenewalForcesRekey

If set to
``yes''
then renewal of the client's GSSAPI credentials will force the rekeying of the
ssh connection. With a compatible server, this can delegate the renewed
credentials to a session on the server.
The default is
``no''

GSSAPITrustDns

Set to
``yes''
to indicate that the DNS is trusted to securely canonicalize
the name of the host being connected to. If
``no''
the hostname entered on the
command line will be passed untouched to the GSSAPI library.
The default is
``no''

HashKnownHosts

Indicates that
ssh(1)
should hash host names and addresses when they are added to
~/.ssh/known_hosts
These hashed names may be used normally by
ssh(1)
and
sshd(8),
but they do not reveal identifying information should the file's contents
be disclosed.
The default is
``no''
Note that existing names and addresses in known hosts files
will not be converted automatically,
but may be manually hashed using
ssh-keygen1.
Use of this option may break facilities such as tab-completion that rely
on being able to read unhashed host names from
~/.ssh/known_hosts

HostbasedAuthentication

Specifies whether to try rhosts based authentication with public key
authentication.
The argument must be
``yes''
or
``no''
The default is
``no''

HostbasedKeyTypes

Specifies the key types that will be used for hostbased authentication
as a comma-separated pattern list.
Alternately if the specified value begins with a
`+'
character, then the specified key types will be appended to the default set
instead of replacing them.
The default for this option is:

Specifies the host key algorithms
that the client wants to use in order of preference.
Alternately if the specified value begins with a
`+'
character, then the specified key types will be appended to the default set
instead of replacing them.
The default for this option is:

If hostkeys are known for the destination host then this default is modified
to prefer their algorithms.

The list of available key types may also be obtained using the
-Q
option of
ssh(1)
with an argument of
``key''

HostKeyAlias

Specifies an alias that should be used instead of the
real host name when looking up or saving the host key
in the host key database files.
This option is useful for tunneling SSH connections
or for multiple servers running on a single host.

HostName

Specifies the real host name to log into.
This can be used to specify nicknames or abbreviations for hosts.
If the hostname contains the character sequence
`%h'
,
then this will be replaced with the host name specified on the command line
(this is useful for manipulating unqualified names).
The character sequence
`%%'
will be replaced by a single
`%'
character, which may be used when specifying IPv6 link-local addresses.

The default is the name given on the command line.
Numeric IP addresses are also permitted (both on the command line and in
HostName
specifications).

IdentitiesOnly

Specifies that
ssh(1)
should only use the authentication identity and certificate files explicitly
configured in the
~/.ssh/config
files
or passed on the
ssh(1)
command-line,
even if
ssh-agent1
or a
PKCS11Provider
offers more identities.
The argument to this keyword must be
``yes''
or
``no''
This option is intended for situations where ssh-agent
offers many different identities.
The default is
``no''

IdentityFile

Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
identity is read.
The default is
~/.ssh/identity
for protocol version 1, and
~/.ssh/id_dsa
~/.ssh/id_ecdsa
~/.ssh/id_ed25519
and
~/.ssh/id_rsa
for protocol version 2.
Additionally, any identities represented by the authentication agent
will be used for authentication unless
IdentitiesOnly
is set.
If no certificates have been explicitly specified by
CertificateFilessh(1)
will try to load certificate information from the filename obtained by
appending
-cert.pub
to the path of a specified
IdentityFile

The file name may use the tilde
syntax to refer to a user's home directory or one of the following
escape characters:
`%d'
(local user's home directory),
`%u'
(local user name),
`%l'
(local host name),
`%h'
(remote host name) or
`%r'
(remote user name).

It is possible to have
multiple identity files specified in configuration files; all these
identities will be tried in sequence.
Multiple
IdentityFile
directives will add to the list of identities tried (this behaviour
differs from that of other configuration directives).

IdentityFile
may be used in conjunction with
IdentitiesOnly
to select which identities in an agent are offered during authentication.
IdentityFile
may also be used in conjunction with
CertificateFile
in order to provide any certificate also needed for authentication with
the identity.

IgnoreUnknown

Specifies a pattern-list of unknown options to be ignored if they are
encountered in configuration parsing.
This may be used to suppress errors if
~/.ssh/config
contains options that are unrecognised by
ssh(1).
It is recommended that
IgnoreUnknown
be listed early in the configuration file as it will not be applied
to unknown options that appear before it.

IPQoS

Specifies the IPv4 type-of-service or DSCP class for connections.
Accepted values are
``af11''
``af12''
``af13''
``af21''
``af22''
``af23''
``af31''
``af32''
``af33''
``af41''
``af42''
``af43''
``cs0''
``cs1''
``cs2''
``cs3''
``cs4''
``cs5''
``cs6''
``cs7''
``ef''
``lowdelay''
``throughput''
``reliability''
or a numeric value.
This option may take one or two arguments, separated by whitespace.
If one argument is specified, it is used as the packet class unconditionally.
If two values are specified, the first is automatically selected for
interactive sessions and the second for non-interactive sessions.
The default is
``lowdelay''
for interactive sessions and
``throughput''
for non-interactive sessions.

KbdInteractiveAuthentication

Specifies whether to use keyboard-interactive authentication.
The argument to this keyword must be
``yes''
or
``no''
The default is
``yes''

KbdInteractiveDevices

Specifies the list of methods to use in keyboard-interactive authentication.
Multiple method names must be comma-separated.
The default is to use the server specified list.
The methods available vary depending on what the server supports.
For an OpenSSH server,
it may be zero or more of:
``bsdauth''
``pam''
and
``skey''

KexAlgorithms

Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated.
Alternately if the specified value begins with a
`+'
character, then the specified methods will be appended to the default set
instead of replacing them.
The default is:

The list of available key exchange algorithms may also be obtained using the
-Q
option of
ssh(1)
with an argument of
``kex''

LocalCommand

Specifies a command to execute on the local machine after successfully
connecting to the server.
The command string extends to the end of the line, and is executed with
the user's shell.
The following escape character substitutions will be performed:
`%d'
(local user's home directory),
`%h'
(remote host name),
`%l'
(local host name),
`%n'
(host name as provided on the command line),
`%p'
(remote port),
`%r'
(remote user name) or
`%u'
(local user name) or
`%C'
by a hash of the concatenation: %l%h%p%r.

The command is run synchronously and does not have access to the
session of the
ssh(1)
that spawned it.
It should not be used for interactive commands.

This directive is ignored unless
PermitLocalCommand
has been enabled.

LocalForward

Specifies that a TCP port on the local machine be forwarded over
the secure channel to the specified host and port from the remote machine.
The first argument must be
[bind_address : port
]
and the second argument must be
host : hostport
IPv6 addresses can be specified by enclosing addresses in square brackets.
Multiple forwardings may be specified, and additional forwardings can be
given on the command line.
Only the superuser can forward privileged ports.
By default, the local port is bound in accordance with the
GatewayPorts
setting.
However, an explicit
bind_address
may be used to bind the connection to a specific address.
The
bind_address
of
``localhost''
indicates that the listening port be bound for local use only, while an
empty address or
`*'
indicates that the port should be available from all interfaces.

LogLevel

Gives the verbosity level that is used when logging messages from
ssh(1).
The possible values are:
QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
The default is INFO.
DEBUG and DEBUG1 are equivalent.
DEBUG2 and DEBUG3 each specify higher levels of verbose output.

MACs Specifies the MAC (message authentication code) algorithms

in order of preference.
The MAC algorithm is used for data integrity protection.
Multiple algorithms must be comma-separated.
If the specified value begins with a
`+'
character, then the specified algorithms will be appended to the default set
instead of replacing them.

The algorithms that contain
``-etm''
calculate the MAC after encryption (encrypt-then-mac).
These are considered safer and their use recommended.

The list of available MAC algorithms may also be obtained using the
-Q
option of
ssh(1)
with an argument of
``mac''

NoHostAuthenticationForLocalhost

This option can be used if the home directory is shared across machines.
In this case localhost will refer to a different machine on each of
the machines and the user will get many warnings about changed host keys.
However, this option disables host authentication for localhost.
The argument to this keyword must be
``yes''
or
``no''
The default is to check the host key for localhost.

NumberOfPasswordPrompts

Specifies the number of password prompts before giving up.
The argument to this keyword must be an integer.
The default is 3.

PasswordAuthentication

Specifies whether to use password authentication.
The argument to this keyword must be
``yes''
or
``no''
The default is
``yes''

PermitLocalCommand

Allow local command execution via the
LocalCommand
option or using the
! command
escape sequence in
ssh(1).
The argument must be
``yes''
or
``no''
The default is
``no''

PKCS11Provider

Specifies which PKCS#11 provider to use.
The argument to this keyword is the PKCS#11 shared library
ssh(1)
should use to communicate with a PKCS#11 token providing the user's
private RSA key.

Port

Specifies the port number to connect on the remote host.
The default is 22.

PreferredAuthentications

Specifies the order in which the client should try authentication methods.
This allows a client to prefer one method (e.g.
keyboard-interactive
over another method (e.g.
password )
The default is:

gssapi-with-mic,hostbased,publickey,
keyboard-interactive,password

Protocol

Specifies the protocol versions
ssh(1)
should support in order of preference.
The possible values are
`1'
and
`2'
Multiple versions must be comma-separated.
When this option is set to
``2,1''
ssh
will try version 2 and fall back to version 1
if version 2 is not available.
The default is
`2'
Protocol 1 suffers from a number of cryptographic weaknesses and should
not be used.
It is only offered to support legacy devices.

ProxyCommand

Specifies the command to use to connect to the server.
The command
string extends to the end of the line, and is executed
using the user's shell
`exec'
directive to avoid a lingering shell process.

In the command string, any occurrence of
`%h'
will be substituted by the host name to
connect,
`%p'
by the port, and
`%r'
by the remote user name.
The command can be basically anything,
and should read from its standard input and write to its standard output.
It should eventually connect an
sshd(8)
server running on some machine, or execute
sshd -i
somewhere.
Host key management will be done using the
HostName of the host being connected (defaulting to the name typed by
the user).
Setting the command to
``none''
disables this option entirely.
Note that
CheckHostIP
is not available for connects with a proxy command.

This directive is useful in conjunction with
nc(1)
and its proxy support.
For example, the following directive would connect via an HTTP proxy at
192.0.2.0:

ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p

ProxyUseFdpass

Specifies that
ProxyCommand
will pass a connected file descriptor back to
ssh(1)
instead of continuing to execute and pass data.
The default is
``no''

PubkeyAcceptedKeyTypes

Specifies the key types that will be used for public key authentication
as a comma-separated pattern list.
Alternately if the specified value begins with a
`+'
character, then the key types after it will be appended to the default
instead of replacing it.
The default for this option is:

Specifies whether to try public key authentication.
The argument to this keyword must be
``yes''
or
``no''
The default is
``yes''

RekeyLimit

Specifies the maximum amount of data that may be transmitted before the
session key is renegotiated, optionally followed a maximum amount of
time that may pass before the session key is renegotiated.
The first argument is specified in bytes and may have a suffix of
`K'
`M'
or
`G'
to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
The default is between
`1G'
and
`4G'
depending on the cipher.
The optional second value is specified in seconds and may use any of the
units documented in the
TIME FORMATS section of
sshd_config5.
The default value for
RekeyLimit
is
``default none''
which means that rekeying is performed after the cipher's default amount
of data has been sent or received and no time based rekeying is done.

RemoteForward

Specifies that a TCP port on the remote machine be forwarded over
the secure channel to the specified host and port from the local machine.
The first argument must be
[bind_address : port
]
and the second argument must be
host : hostport
IPv6 addresses can be specified by enclosing addresses in square brackets.
Multiple forwardings may be specified, and additional
forwardings can be given on the command line.
Privileged ports can be forwarded only when
logging in as root on the remote machine.

If the
port
argument is
`0'
,
the listen port will be dynamically allocated on the server and reported
to the client at run time.

If the
bind_address
is not specified, the default is to only bind to loopback addresses.
If the
bind_address
is
`*'
or an empty string, then the forwarding is requested to listen on all
interfaces.
Specifying a remote
bind_address
will only succeed if the server's
GatewayPorts
option is enabled (see
sshd_config5).

RequestTTY

Specifies whether to request a pseudo-tty for the session.
The argument may be one of:
``no''
(never request a TTY),
``yes''
(always request a TTY when standard input is a TTY),
``force''
(always request a TTY) or
``auto''
(request a TTY when opening a login session).
This option mirrors the
-t
and
-T
flags for
ssh(1).

RevokedHostKeys

Specifies revoked host public keys.
Keys listed in this file will be refused for host authentication.
Note that if this file does not exist or is not readable,
then host authentication will be refused for all hosts.
Keys may be specified as a text file, listing one public key per line, or as
an OpenSSH Key Revocation List (KRL) as generated by
ssh-keygen1.
For more information on KRLs, see the KEY REVOCATION LISTS section in
ssh-keygen1.

RhostsRSAAuthentication

Specifies whether to try rhosts based authentication with RSA host
authentication.
The argument must be
``yes''
or
``no''
The default is
``no''
This option applies to protocol version 1 only and requires
ssh(1)
to be setuid root.

RSAAuthentication

Specifies whether to try RSA authentication.
The argument to this keyword must be
``yes''
or
``no''
RSA authentication will only be
attempted if the identity file exists, or an authentication agent is
running.
The default is
``yes''
Note that this option applies to protocol version 1 only.

SendEnv

Specifies what variables from the local
environ(7)
should be sent to the server.
The server must also support it, and the server must be configured to
accept these environment variables.
Note that the
TERM
environment variable is always sent whenever a
pseudo-terminal is requested as it is required by the protocol.
Refer to
AcceptEnv
in
sshd_config5
for how to configure the server.
Variables are specified by name, which may contain wildcard characters.
Multiple environment variables may be separated by whitespace or spread
across multiple
SendEnv
directives.
The default is not to send any environment variables.

See
Sx PATTERNS
for more information on patterns.

ServerAliveCountMax

Sets the number of server alive messages (see below) which may be
sent without
ssh(1)
receiving any messages back from the server.
If this threshold is reached while server alive messages are being sent,
ssh will disconnect from the server, terminating the session.
It is important to note that the use of server alive messages is very
different from
TCPKeepAlive
(below).
The server alive messages are sent through the encrypted channel
and therefore will not be spoofable.
The TCP keepalive option enabled by
TCPKeepAlive
is spoofable.
The server alive mechanism is valuable when the client or
server depend on knowing when a connection has become inactive.

The default value is 3.
If, for example,
ServerAliveInterval
(see below) is set to 15 and
ServerAliveCountMax
is left at the default, if the server becomes unresponsive,
ssh will disconnect after approximately 45 seconds.

ServerAliveInterval

Sets a timeout interval in seconds after which if no data has been received
from the server,
ssh(1)
will send a message through the encrypted
channel to request a response from the server.
The default
is 0, indicating that these messages will not be sent to the server,
or 300 if the
BatchMode
option is set.
ProtocolKeepAlives
and
SetupTimeOut
are Debian-specific compatibility aliases for this option.

StreamLocalBindMask

Sets the octal file creation mode mask
(umask)
used when creating a Unix-domain socket file for local or remote
port forwarding.
This option is only used for port forwarding to a Unix-domain socket file.

The default value is 0177, which creates a Unix-domain socket file that is
readable and writable only by the owner.
Note that not all operating systems honor the file mode on Unix-domain
socket files.

StreamLocalBindUnlink

Specifies whether to remove an existing Unix-domain socket file for local
or remote port forwarding before creating a new one.
If the socket file already exists and
StreamLocalBindUnlink
is not enabled,
ssh
will be unable to forward the port to the Unix-domain socket file.
This option is only used for port forwarding to a Unix-domain socket file.

The argument must be
``yes''
or
``no''
The default is
``no''

StrictHostKeyChecking

If this flag is set to
``yes''
ssh(1)
will never automatically add host keys to the
~/.ssh/known_hosts
file, and refuses to connect to hosts whose host key has changed.
This provides maximum protection against trojan horse attacks,
though it can be annoying when the
/etc/ssh/ssh_known_hosts
file is poorly maintained or when connections to new hosts are
frequently made.
This option forces the user to manually
add all new hosts.
If this flag is set to
``no''
ssh will automatically add new host keys to the
user known hosts files.
If this flag is set to
``ask''
new host keys
will be added to the user known host files only after the user
has confirmed that is what they really want to do, and
ssh will refuse to connect to hosts whose host key has changed.
The host keys of
known hosts will be verified automatically in all cases.
The argument must be
``yes''
``no''
or
``ask''
The default is
``ask''

TCPKeepAlive

Specifies whether the system should send TCP keepalive messages to the
other side.
If they are sent, death of the connection or crash of one
of the machines will be properly noticed.
This option only uses TCP keepalives (as opposed to using ssh level
keepalives), so takes a long time to notice when the connection dies.
As such, you probably want
the
ServerAliveInterval
option as well.
However, this means that
connections will die if the route is down temporarily, and some people
find it annoying.

The default is
``yes''
(to send TCP keepalive messages), and the client will notice
if the network goes down or the remote host dies.
This is important in scripts, and many users want it too.

To disable TCP keepalive messages, the value should be set to
``no''

Tunnel

Request
tun(4)
device forwarding between the client and the server.
The argument must be
``yes''
``point-to-point''
(layer 3),
``ethernet''
(layer 2),
or
``no''
Specifying
``yes''
requests the default tunnel mode, which is
``point-to-point''
The default is
``no''

TunnelDevice

Specifies the
tun(4)
devices to open on the client
(local_tun
)
and the server
(remote_tun
)

The argument must be
local_tun [: remote_tun
]
The devices may be specified by numerical ID or the keyword
``any''
which uses the next available tunnel device.
If
remote_tun
is not specified, it defaults to
``any''
The default is
``any:any''

UpdateHostKeys

Specifies whether
ssh(1)
should accept notifications of additional hostkeys from the server sent
after authentication has completed and add them to
UserKnownHostsFile
The argument must be
``yes''
``no''
(the default) or
``ask''
Enabling this option allows learning alternate hostkeys for a server
and supports graceful key rotation by allowing a server to send replacement
public keys before old ones are removed.
Additional hostkeys are only accepted if the key used to authenticate the
host was already trusted or explicitly accepted by the user.
If
UpdateHostKeys
is set to
``ask''
then the user is asked to confirm the modifications to the known_hosts file.
Confirmation is currently incompatible with
ControlPersist
and will be disabled if it is enabled.

Presently, only
sshd(8)
from OpenSSH 6.8 and greater support the
``hostkeys@openssh.com''
protocol extension used to inform the client of all the server's hostkeys.

UsePrivilegedPort

Specifies whether to use a privileged port for outgoing connections.
The argument must be
``yes''
or
``no''
The default is
``no''
If set to
``yes''
ssh(1)
must be setuid root.
Note that this option must be set to
``yes''
for
RhostsRSAAuthentication
with older servers.

User

Specifies the user to log in as.
This can be useful when a different user name is used on different machines.
This saves the trouble of
having to remember to give the user name on the command line.

UserKnownHostsFile

Specifies one or more files to use for the user
host key database, separated by whitespace.
The default is
~/.ssh/known_hosts
~/.ssh/known_hosts2

VerifyHostKeyDNS

Specifies whether to verify the remote key using DNS and SSHFP resource
records.
If this option is set to
``yes''
the client will implicitly trust keys that match a secure fingerprint
from DNS.
Insecure fingerprints will be handled as if this option was set to
``ask''
If this option is set to
``ask''
information on fingerprint match will be displayed, but the user will still
need to confirm new host keys according to the
StrictHostKeyChecking
option.
The argument must be
``yes''
``no''
or
``ask''
The default is
``no''

If this flag is set to
``yes''
an ASCII art representation of the remote host key fingerprint is
printed in addition to the fingerprint string at login and
for unknown host keys.
If this flag is set to
``no''
no fingerprint strings are printed at login and
only the fingerprint string will be printed for unknown host keys.
The default is
``no''

XAuthLocation

Specifies the full pathname of the
xauth(1)
program.
The default is
/usr/bin/xauth

PATTERNS

A
pattern
consists of zero or more non-whitespace characters,
`*'
(a wildcard that matches zero or more characters),
or
`?'
(a wildcard that matches exactly one character).
For example, to specify a set of declarations for any host in the
``.co.uk''
set of domains,
the following pattern could be used:

Host *.co.uk

The following pattern
would match any host in the 192.168.0.[0-9] network range:

Host 192.168.0.?

A
pattern-list
is a comma-separated list of patterns.
Patterns within pattern-lists may be negated
by preceding them with an exclamation mark
(`!'
)
For example,
to allow a key to be used from anywhere within an organization
except from the
``dialup''
pool,
the following entry (in authorized_keys) could be used:

from="!*.dialup.example.com,*.example.com"

FILES

~/.ssh/config

This is the per-user configuration file.
The format of this file is described above.
This file is used by the SSH client.
Because of the potential for abuse, this file must have strict permissions:
read/write for the user, and not accessible by others.
It may be group-writable provided that the group in question contains only
the user.

/etc/ssh/ssh_config

Systemwide configuration file.
This file provides defaults for those
values that are not specified in the user's configuration file, and
for those users who do not have a configuration file.
This file must be world-readable.

SEE ALSO

AUTHORS

OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.
Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
Theo de Raadt and Dug Song
removed many bugs, re-added newer features and
created OpenSSH.
Markus Friedl contributed the support for SSH
protocol versions 1.5 and 2.0.